Segmented pipe couplings are well-known in the art, and are comprised of two or more arcuate coupling segments which are assembled in end-to-end encircling relationship with the ends of pipes or fittings which have been positioned in end-to-end relationship and substantially in axial alignment.
The coupling segments are secured to each other at their adjacent ends by track bolts extending through bolting pads at the ends of the coupling segments, or, by any other convenient means. On tightening down of the track bolts the coupling segments are drawn radially inwardly of the axis of the pipes, for the coupling segments either to come into clamping engagement with the external peripheries of the pipes, or, for keys on the inner peripheries of the coupling segments to be moved into engagement with formations on the pipes, such as grooves in the pipes, or, radial excrescences formed or provided exteriorly thereof.
A requirement of such pipe couplings is that they be provided interiorly with a sealing gasket, in order to prevent leakage from the pipes at the adjacent end thereof, and axially past the coupling segments, or, radially between the bolting faces thereof.
Such gaskets commonly are in the form of split gaskets, such that they can be placed over the pipe peripheries, as opposed to being forced axially thereof, and commonly are formed for them to be positioned within and be held captive by the respective coupling segments prior to the assembly of the coupling.
On assembly of the coupling, the ends of the gasket segments are brought into compressive engagement with the ends of adjacent gasket segments, and, into compressive engagement with the pipe peripheries, this occurring during tightening down of the coupling segments.
As a result of tightening down of the coupling segments, compressive hoop stresses are produced in the gasket segments, which cause the respective gasket segments to retract circumferentially into the coupling segments, this acting to equalize the compressive hoop stress throughout the circumferential extent of the gasket segment.
While this equalization of the compressive hoop stress is of assistance in providing uniform compressive sealing engagement of the gasket segment with the pipe peripheries, it also has the disadvantageous result of a reduction in the compressive stress exerted at the end faces of the gasket segment on the end faces of the next adjacent gasket segment.
Thus, as the end faces of the respective gasket segments are located intermediate the bolting faces of the adjacent coupling segments, a loss in compressive sealing engagement can occur at that location, which is exactly that location at which the compressive stresses optimally should be at the possible maximum in order to prevent leakage at that location and radially outwardly between the bolting faces of the adjacent coupling segments.
This problem is further exacerbated in the event that the coupling is of the "rigid" type, which clamps directly onto the pipe peripheries. In this situation, any deviation of the pipe diameter from its standard size, such as invariably occurs due to manufacturing tolerances, can result either in pinching of the gasket material in the event that the pipe is undersized, or, and more importantly, a substantial loss in compressive sealing force exerted between the adjacent end faces of the gasket segments in the event that the pipes are over-sized, and, consequential leakage at that location.
This problem is further exacerbated in the event that one of the pipes is oversized and the other is undersized, in that both of the conditions discussed above can occur in combination with each other.
A partial solution to this problem is taught in Blakeley, U.S. Pat. No. 4,391,458 issued July 5, 1983, which teaches the formation of the ends of gasket segments with radially outwardly extending thickened portions which engage behind internal shoulders on his coupling segments. Blakeley's approach is, however, only partially successful in dealing with conditions arising from oversizing of the pipes or the undersizing thereof, or, a combination of both such conditions in a single pipe joint.
Recognizing this problem, Blakeley resorts to the off-setting of the ends of the gasket segments relative to the bolting faces of his coupling segments, such that the plane of the abutting ends of his gasket segments is displaced angularly with respect to the plane of the bolting faces of his coupling segments. This, however, of essence requires an asymmetrical configuration of the gasket segments, with a likelihood that a workman will insert a gasket segment into the coupling segment in a reversed position, this resulting in over-compression of the gasket segments at one end thereof, and, zero compression and possible actual spacing of the gasket ends at the opposite ends thereof.